Directional boring, commonly called horizontal directional drilling is a steerable trenchless method of installing underground pipes, conduits and cables, well screens or the like in a shallow arc, underground typically along a prescribed bore path by using a surface launched drilling rig. Pipes laid, or well casing installed in this manner can be made of materials such as iron, steel, PVC, polyethylene, polypropylene, or the like. With this type of drilling there is typically minimal impact on the surrounding area compared to using a trencher or other trench-based installation methods. Directional boring can often be used when trenching or excavating is not practical, such as under roadways, or other existing structures, or at greater depths than it is possible to trench. It is suitable for a variety of soil conditions and jobs that may include road, landscape, environmental remediation and monitoring or water production well applications.
The use of horizontal or directionally drilled wells has become widespread for treatment of contaminated soil or groundwater, construction dewatering, drainage, and water production. In many situations, horizontal wells have advantages over vertical wells for these purposes. For construction dewatering or drainage, horizontal wells can provide extended coverage with very little impact on surface operations, compared to comparable networks of vertical wells. For groundwater production, horizontal wells can capture groundwater from thin or sinuous aquifers that are difficult to tap with vertical wells. For the treatment of contaminated soil or groundwater, horizontal wells offer the advantages of access beneath surface obstructions, such as buildings or sensitive wetlands that would prevent the installation of vertical wells to treat the area. Other advantages also exist.
In installing horizontal and vertical wells, at the end of the bore the well casing extends into a zone that allows liquid, such as groundwater, to be collected and enter the well. Typically a structure is provided that tends to allow the liquid in the surrounding ground to flow into the well bore, without the flow causing the bore to clog. The structure also tends to aid in the flow of fluid into the bore, effectively increasing the flow coming out of the well.
Part of the structure at the end of the bore is a sand pack. One disadvantage of horizontal wells has been the difficulty in the construction of a traditional “sand pack” surrounding the well screen in the horizontal well, as the techniques used in a vertical well causes problems when applied to a vertical well.
FIG. 1 shows a conventional vertical well 100. In a vertical well a screen 112 may be used to keep large particles out of the well pipe, pump, and to prevent clogging of the well pipe 114.
The well screen, which is commonly constructed of carbon or stainless steel, Poly Vinyl Chloride (PVC), High Density Polyethylene (HDPE), or other material provides the component through which water enters the well. During installation the well screen may be suspended in a vertical boring 116 which is of a larger diameter than the screen. The space between the screen and the walls of the bore, known as the well annulus, may be backfilled with sand or gravel 110 during the construction of the well.
Upon development of the well by pumping or other techniques, fine material is washed by liquid 108 from the granular backfill and a graded sand pack is formed through the washing action 118. The sand pack 110 typically includes coarser material adjacent to the well screen, grading to finer material near the wall of the boring. Proper sizing of the well screen slots or perforations, combined with proper selection of the appropriate mesh size of the backfill granular material, results in well that does not entrain fine clay, silt, or sand into the well after development is completed.
The completion of a sand pack, as described for a vertical well, relies on gravity to place the granular backfill. The well screen is held at a central position within the bore using centralizers or some similar mechanism 120.
A centralizer 120 is a device that may include three or more spring-steel protrusions that can be clamped to the well screen as it is lowered into the vertical bore. The centralizers contact the borehole and hold the well screen in the center of the bore. The granular backfill (appropriately sized sand) is then simply distributed evenly around the well screen until it covers the screened interval in the well. Bentonite seals and/or plugs may then be installed past the fill line in the bore 122, followed by a grout slurry to the ground surface.
Horizontal wells present a unique problem in providing sufficient filtering that would be provided by a conventional well screen assembly used in a vertical well. For example the use of centralizers in a meandering, horizontal well path is problematic. Also producing a sand pack in a horizontal path, without aid of gravity to direct the sand to the location of the screen is problematic.